Rehabilitation under way on Cape Flats 1 sewer line

REHABILITATION UNDER WAY ON

CAPE FLATS 1 SEWER LINE

The Cape Flats 1 and 2 (CF 1 and 2) sewer lines were originally installed in 1962 and 1969 respectively. Now a major refurbishment programme is under way in stages. Alastair Currie speaks to Vuyo James, Pr Tech Eng, Senior Professional Officer: Planning, Design and Projects at the City of Cape Town, about the trenchless technique employed.

Demonstration prior to commencement of works on Cape Flats 1

Based on recent pipe profiling inspections, what is the current overall condition of the CF 1 and 2 lines?

VJ For background information, Athlone is the starting point for CF 1 and 2 – each measuring approximately 14 km in length – culminating at the Cape Flats Wastewater Treatment Works in Zeekoevlei. A further line, Cape Flats 3 (CF 3), was completed in 2016 to meet ongoing population growth and industrial expansion.

Given their age, CF 1 and 2 have stood up remarkably well over the years, but it’s time for an intensive maintenance intervention to further extend their working life. Thanks to the addition of CF 3, we can embark on a systematic and phased refurbishment of CF 1 and 2 without comprising on sanitation delivery. The refurbishment programme starts with CF 1, where pipe diameters range from 1 050 mm to 1 430 mm. The option of SWP fixed-diameter technology is being used, as this provides the City with the best solution in terms of the use of cleaning equipment and the lifespan of the rehabilitated pipeline.

The major benefit of SWP is that it can be installed under live conditions. Where practical, it will be used for the whole of CF 1.

What are the project costs, phases and timelines?

The overall CF 1 project has been split into multiple works packages spanning a tender term of three years. Due to the unknown conditions of the pipeline at the commencement of the project, the CF 1 rehabilitation works have been split into two main phases, namely: cleaning, inspection and profiling; and SWP rehabilitation.

Our first section on CF 1 needed to be completed ahead of a City Integrated Rapid Transit project commencing in Govan Mbeki Road on 1 July 2022.

The length of this 1 100 mm diameter CF 1 section is approximately 1.2 km starting at Pinati Estate in Athlone and extending to Govan Mbeki Road in Wetton. The works began in mid-April 2022 for scheduled completion by the end of June 2022. The approximate cost is R27 000 per metre inclusive of cleaning, all civil works and rehabilitation.

Based on the contractor’s profiling of the pipelines, it was shown that the pipelines have a severely corroded soffit. From these profiling inspections, it was determined that the pipeline ovality varies from between 2.5% to approximately 14%. It was determined, however, that the pipelines are only partially deteriorated, as the severely corroded sections of pipe were encased in concrete at the time of construction. The corrosion of the pipelines has an impact on their flow condition and the high potential of collapse, plus the requirement for point repairs in the future.

Which trenchless technology repair interventions have been approved by the City and why?

Spirally wound pipe (SWP) lining is the preferred technology to be used for the rehabilitation of CF 1. This is based on the fact that the SWP liner caters for a wide range of deterioration conditions with a single lining profile thickness.

This allows for a lower risk to the City and contractor in ordering material in advance, as this material satisfies the design requirements for multiple conditions with a high safety factor.

Vuyo James, Senior Professional Officer: Planning, Design and Projects, Water and Sanitation Directorate, City of Cape Town

SWP launch preparation

Will the manhole chambers be replaced?

On this particular project, the existing manhole chambers, which are currently 650 mm x 650 mm brick chimneys, are being upgraded to 1 250 mm diameter precast manhole ring chimneys. The chambers below the cover slabs remain unchanged except for minimal repairs required for the installation of the SWP liner.

What are some of the key technical challenges?

Due to the high ovality of the pipeline, considerable effort went into the design and proposal of the fixeddiameter SWP liner in order to improve flow conditions and rehabilitate the pipeline without significantly reducing the cross-sectional area of the existing pipeline.

Has SWP been used by the City before?

Prior to this contract, SWP liner rehabilitation had never been implemented within the City of Cape Town municipal area, making this contract a flagship project for trenchless technology for large-diameter gravity sewer pipeline rehabilitation.

The City of Cape Town has previously implemented cured-in-place-pipe (CIPP) lining technology. This provided the City with in-depth experience on the implementation of large-scale pipeline rehabilitation projects.

The use of the new SWP technology has the advantage of lower risk of installation in comparison with alternative methods and a reduced construction timeframe due to the ease of installation. This minimises the disruption to the surrounding/affected communities. An added benefit is that the SWP liner is designed to have a lifespan of 100 years.

SWP feed in progress An installed SWP section on Cape Flats 1

ABOUT SWP

Spiral-wound liners are a specialty extruded plastic ribbed profile made from pipe-grade PVC. The liners are constructed directly on-site using mechanical winding machines and the PVC profile is shipped to the job site on spools. The profile strip is wound into the existing pipe via stationary or traversing winding machines, with the edges of the profiles joining to form a continuous liner. The result is a new PVC pipe within the existing pipeline. (Source: www.trenchlesspedia.com)

Will the installation of the new refurbishment solution negate the need for future pipeline maintenance?

No, regular maintenance/cleaning of the pipeline will still be required to mitigate silt settlements in the line. However, the implementation of this rehabilitation method will prevent point repairs/collapses on the line within the designed lifespan of the SWP liner.

What’s the next stage?

Planning starts from mid-June 2022 with the City’s appointed professional service providers. This will determine the next steps and the drawing up of work packages from July for the new CF 1 phases. Based on available budgets, we aim to complete the entire CF 1 rehabilitation by mid-2024.

And in closing?

This is a pioneering project for the rehabilitation of large-diameter gravity pipelines in Southern Africa. With the current state of the existing sewer infrastructure in South Africa, there is a need for the use of trenchless technologies such as SWP.